1. Field of the Invention
This invention relates to the field of Code Division Multiple Access (CDMA) communication systems. More particularly, the present invention relates to a system for accurately detecting short codes in a communication environment which includes continuous wave (CW) interference.
2. Description of Prior Art
With the dramatic increase in the use of wireless telecommunication systems in the past decade, the limited portion of the RF spectrum available for use by such systems has become a critical resource. Wireless communications systems employing CDMA techniques provide an efficient use of the available spectrum by accommodating more users than more traditional time division multiple access (TDMA) and frequency division multiple access (FDMA) systems.
In a CDMA system, the same portion of the frequency spectrum is used for communication by all subscriber units. Typically, for each geographical area, a single base station serves a plurality of subscriber units. The baseband data signal within each subscriber unit is multiplied by a pseudo-random code sequence, called the spreading code, which has a much higher transmission rate than the data. Thus, the data signal is spread over the entire available bandwidth. Individual subscriber unit communications are discriminated by assigning a unique spreading code to each communication link.
At times it is also useful in a CDMA system to transmit codes which are of shorter length than the usual spreading code. Instead of using a single, extremely long spreading code, a much shorter code is used and repeated numerous times. The use of short codes provides an advantage over the use of longer codes because the short codes can be detected much more quickly. However, the use of short codes has an inherent drawback, since the short code is repeated many times, it is much less random than a long code. When short codes are used, known detection algorithms can have an increased number of false acquisitions in the presence of continuous wave (CW) interference since the repetitive short codes can correlate with CW interference.
When there is correlation between short codes and CW interference a false acquisition occurs, an incorrect output from a short code detector in a base station can last for a time period equal to the remainder of a short code. For example, in a known prior art short code system, short codes having 195 chips, which are transmitted at a rate of 15 megahertz, repeat for a three millisecond period. At the end of the three millisecond period a new short code is transmitted in the same manner. In such a system it is possible for a detector output to lock up for the remainder of the three millisecond period in response to a false acquisition in the presence of CW interference.
It is known in the art of mobile communication systems which employ CDMA for a base-station receiver to use various detection tests to determine the presence of short codes transmitted by a subscriber unit. One such test known in the art is a sequential probability ratio test (SPRT) detection algorithm. The problem of false detections in the presence of CW interference can occur in detection algorithms such as a SPRT detection algorithm, even though SPRT detection algorithms can be very effective at rejecting noise under other circumstances.
In SPRT detection algorithms, a likelihood ratio is computed and adjusted after each input sample is taken. The repeated adjustments cause the likelihood ratio to increase when a short code is present and decrease when a short code is not present. When the likelihood ratio increases and crosses a predetermined acceptance threshold, a determination is made that a short code is present. When the likelihood ratio decreases and crosses a predetermined rejection threshold, a determination is made that a short code is not present. When the likelihood ratio is between the acceptance and rejection thresholds further samples are taken and further adjustments are made to the likelihood ratio until one of the thresholds is crossed. Thus, the false detection problem can occur in a SPRT detection algorithm when the CW incorrectly causes the likelihood ratio to increase and cross over the acceptance threshold.
It is desirable to provide method for preventing false acquisitions of short codes in the presence of CW interference that does not limit the number of codes available for use within the system.
A method is disclosed for receiving transmitted signals in the presence of CW interference in a communication system that determines the presence of a short code in a received signal by comparing the output of a detector with threshold calculations made in accordance with a sample of a received signal. Such systems include but are not limited to those incorporating a Sequential Probability Ratio Test detection algorithm. The method includes obtaining a first input power value of the received signal at a first sample time and obtaining a second input power value of the received signal at a second sample time. The first and second power values are compared to provide an input sample comparison and the forgoing steps are repeated to provide a plurality of input sample comparisons. The detector threshold is adjusted in accordance with the plurality of sample comparisons.